In recent years, with a background of development of a trend toward high-definition of image quality in inkjet and expansion of a range of application thereof in an industrial use, demands for fine pattern forming and ejection of high viscosity ink grow greater increasingly. When these problems are attempted to be solved by a conventional inkjet recording method, minimization of nozzles and improvement of liquid ejection force for ejecting high viscosity ink are needed, resulting in high drive voltage and extreme cost increase for a head and a device, which has prevented realization of a device that is suited to practical use.
To meet the aforesaid demands, therefore, there has been known, as a technology to eject not only a low viscosity liquid droplet but also a high viscosity liquid drop from a minimized nozzle, the liquid droplet ejection technology of the so-called electrostatic suction method wherein a liquid in a nozzle is charged electrically, and a liquid droplet is ejected by electrostatic suction force caused by an electric field that is formed between the nozzle and various base materials representing a target that receives an impact of the liquid droplet (see Patent Document 1).
However, when a flat liquid ejection head of this kind is used in the liquid droplet ejection technology of the electrostatic suction method, an extent of electric field concentration for a liquid in a nozzle and for a meniscus of a ejection hole portion is low, and it has been necessary to apply extremely high voltage as voltage to be applied between the liquid ejection head and the base materials, for obtaining necessary electrostatic suction force.
Therefore, there has been advancement of development of a liquid droplet ejection device employing the so-called an electric field assist method wherein this liquid droplet ejection technology and a technology to eject a liquid droplet by using pressure caused by a transformation of a piezoelectric element or by generation of bubbles inside a liquid are combined (for example, see Patent Documents 2-5). This electric field assist method is a method wherein a meniscus forming section and electrostatic suction force are used to protrude a liquid meniscus on an orifice of a nozzle, and thereby to enhance electrostatic suction force for the meniscus so that the electrostatic suction force may overcome a surface tension of a liquid to change the meniscus into a liquid droplet to eject it.
Patent Document 1: International Application Publication No. 03/070381 A1
Patent Document 2: Japanese Patent Publication Open to Public Inspection No. H5-104725
Patent Document 3: Japanese Patent Publication Open to Public Inspection No. H5-278212
Patent Document 4: Japanese Patent Publication Open to Public Inspection No. H6-134992
Patent Document 5: Japanese Patent Publication Open to Public Inspection No. 2003-53977
Compared with an inkjet recording method employing a conventional piezoelectric system or a thermal system, in these liquid ejection devices employing the electric field assist method, electrostatic suction force by electric field is not utilized to its maximum level although the ejection efficiency is satisfactory, thus, forming of the meniscus and ejection of a liquid droplet are not conducted efficiently, and when trying to meet the demands for fine pattern forming and ejection of high viscosity ink, drive voltage needs to be higher, resulting in a cost increase of a head and a device in the same way as in the conventional inkjet recording method, which has been a problem. Further, when voltage to be applied is boosted for enhancing electrostatic suction force, dielectric breakdown is caused between a head and base materials, which sometimes makes it impossible to drive the device, which has also been a problem.
When a flat liquid ejection head is used as a liquid ejection head on which a nozzle for ejecting a liquid is provided, in these liquid ejection devices each employing an electric field assist method, there are great advantages that productivity is excellent because of simple structures, and a nozzle is not caught by a wiper in the case of wiping of a ejection surface when a liquid ejection head is cleaned.
However, even in the case of the liquid ejection device employing the electric field assist method wherein pressure is generated by a transformation of a piezoelectric element, to protrude a liquid meniscus on an ejection hole of a nozzle, and electric field is concentrated selectively on the protruded meniscus to eject a liquid by electrostatic suction force, an action to draw out a meniscus by electrostatic suction force for forming a meniscus is poor because of poor electric field concentration, resulting in necessity of applying high voltage on pressure generating portion that is composed of piezoelectric element actuators such as piezoelectric elements, which has been a problem.
Incidentally, in the invention, a flat nozzle, a nozzle plate and a liquid ejection head mean those wherein a protrusion of a nozzle from a ejection surface of the nozzle plate is 30 μm or less, and they mean those wherein a trouble such as damage is not caused in the course of the aforesaid wiping, and a nozzle protrusion is small and no effect of electric field is expected.
In the liquid ejection device employing the electric field assist method for solving problems of this flat liquid ejection head, therefore, a liquid ejection head wherein a nozzle is protruded in a shape of a lightning rod toward the ejection surface side from a nozzle plate of the liquid ejection head, to enhance ejection efficiency of the nozzle by concentrating electric field to the tip of the protrusion of the nozzle, is used in many cases.
However, a large number of nozzles each being in a lightning rod shape having a height of about several tens μm need to be embedded toward the ejection surface side from the nozzle plate of the liquid ejection head, which makes the structure to be complicated, and lowers productivity. Further, there has been a problem of poor operability that embedded nozzles are broken in the course of cleaning of the liquid ejection head.